The present invention generally relates to pulse oximeters, the use thereof, and/or a method for detecting blood oxygenation.
Pulse oximeters typically measure and display various blood flow characteristics including--but not limited thereto--blood oxygen saturation of hemoglobin in arterial blood, the rate of blood pulsations corresponding to the heart rate of the patient, or a perfusion indicator.
Pulse oximeters generally determine the arterial oxygen saturation of hemoglobin (also called SpO2 or SaO2 measurement) by way of a non-invasive technique using two different monochromatic light sources, which are typically formed by LEDs. Normally one of the LEDs emits light in the red wavelength range of about 645 nm and the other one in the infrared wavelength range of 940 nm. The light emitted by both LEDs is transmitted through a predetermined area of the patient's body.
Typically, pulse oximeter systems utilize an oxygen saturation sensing probe which is arranged to be detachably secured to the patient's finger. Usually, the probe has the form of a clip including both light emitting diodes and a light detector. The probe is arranged such that the light of both light emitting diodes having passed the predetermined area of the patient's body is received by a single light detector. An example for a pulse oximeter is the Hewlett Packard Component Monitoring System with the Pulse Oximeter Module, the `HP M1020A`.
As it is known in the art of pulse oximetry, the light of both light sources is attenuated by static and dynamic absorbers on its path through the patient's body to the light detector. The arterial blood whose quantity varies with the time synchronously with the patient's heartbeat represents the only dynamic absorber during the pulse period. All other absorbers, such as skin, tissue or bone, are not time-variant. Thus, pulse oximeters make use of the pulsatile component of arterial blood generated by the heartbeat at only two spectral lines.
The light detector, which may have the form of a photo diode, receives the modulated light intensities of each wavelength. Then, these signals are usually amplified, low pass filtered, converted from analog to digital and further processed, e.g., in a microprocessor system. A pulse finding algorithm analyses the received signals which are so-called spectrophotometric signals for identifying the pulses and for determining the pulse. After identifying the pulse period, the microprocessor system determines the diastolic and systolic values of the spectrophotometric signals and derives therefrom the so-called relative absorption ratios. Subsequently, the microprocessor system computes in a saturation calculation algorithm the arterial oxygen saturation from the relative absorption ratio using calibration data and so-called extinction coefficients from the absorption spectrum of hemoglobin and oxyhemoglobin at the appropriate wavelengths. The mathematical background therefor, which makes use of Lambert-Beer's law, has been described in sufficient detail in a multiplicity of former publications. See, for example, EP-A-262 778 which contains a rather good breakdown of the theory.
Since the early 1980s, when pulse oximetry was introduced, this non-invasive method of monitoring the arterial oxygen saturation level in a patient's blood (SpO2) has become a standard method in the clinical environment because of its simple application and the high value of the information applicable to nurses and doctors. It has become as common in patient monitoring to measure the oxygen level in the blood as to monitor heart activity with the ECG. In some application areas, like anesthesia in a surgical procedure, it is mandatory for doctors to measure this vital parameter.
Background information about pulse oximetry is given by S. Kastle et al., "A New Family of Sensors for Pulse Oximetry", Hewlett-Packard Journal, February 1997, pages 39-53.
In recent time, patient monitoring has been expanded from pure stationary monitoring with stationary instruments, e.g., on intensive care units, to non-stationary monitoring comprising smaller and mobile instruments such as telemetry or handhold instruments. Telemetry systems, such as an Hewlett Packard `HP M1403A Digital Telemetry System`, consist of a telemetric transmitter (like an Hewlett Packard `HP M1400A pocket sized UHF Transmitter`) with embedded measurement devices, such as ECG or SpO2. The transmitter is battery driven and carried around by a stationary or ambulating patient. The measurement signal is transmitted via UHF to a central UHF receiver unit (like an Hewlett Packard `HP M1401A` mainframe), having one receiving channel for each transmitter. The received information is passed to a central display unit (like an Hewlett Packard `HP 78560A Central Monitor`), where the information is displayed.
SpO2 measurement systems require a relatively high amount of electrical power. Therefore, SpO2 measuring has found only limited access to telemetric or handhold instruments, because in such a measurement environment, power is generally restricted to the battery life time.
One approach to employ SpO2 measurement while telemeter or handhold instruments are used is to simply measure SpO2 by separate stationary units, however, only manually and on demand. This approach cannot satisfy the demands of safe patient monitoring required in several applications.
Another approach to implement SpO2 measurement in telemetric or handhold instruments has been by using larger batteries. However, those large capacity batteries tend to be relatively costly and also heavy in weight, thus having a negative impact on the usability of those telemetry or handhold instruments.